Constant velocity joints are used in numerous vehicular applications where the rotational velocity oscillation of a conventional cardan joint is unacceptable. For example, in the front suspension of a front wheel drive automobile, there will be two constant velocity joints per axle. They are also used in off-road heavy-duty equipment, in trucks, and in high performance recreation vehicles.
When the application is not overly environmentally adverse, constant velocity joints are excellent. However, in environmentally unfriendly applications they are less desirable because of the problems of keeping dirt and debris out of the joint. A better understanding of that will be appreciated upon review of FIG. 1 which shows a conventional prior art constant velocity joint.
FIG. 1 is, as will be apparent to those skilled in the art, a diagram of a constant velocity joint. An input shaft 21 is attached to a housing 22 which is formed to have a plurality of ball races 23. An inner race member 25 also includes a plurality of ball races, and a plurality of balls 26 connect the two members 25, 22 by means of the balls 26 riding in pairs of the associated races. A cage 28 encircles the balls 26 and keeps them in a constant velocity plane as the joint flexes. Typically the inner race 25 is splined at 30 and receives a shaft 32 splined at 31. By its very nature, the end 40 of the constant velocity joint through which all of the components are assembled is open. A primary seal is provided by boot 35. It is usually a fairly large and flexible member to accommodate the movement of the joint. Thus it is usually made of relatively flexible rubber and is fixed by a ring 36 to the outside of the housing and by another ring 37 to the outside of the shaft 32. A bellows area 38 allows for flexing of the boot 35 as the angle of the output shaft 32 changes with respect to the input shaft 21.
In the normal automotive environment a boot of this type can protect the joint for many thousands of miles of operation. However, as has become apparent to some motorists, once the boot tears, debris and moisture can enter the joint because the boot itself is the primary seal. It is not, like in conventional cardan joints, a simple secondary dust shield, but is the primary seal for keeping foreign material out of the workings of the joint mechanism. Thus, when the boot tears, it is not long thereafter, without attention, that the joint will fail.
Although it is desirable to use constant velocity joints in more environmentally demanding applications, the inability of the seal to withstand tough environmental conditions is a strong negative factor. In off the road applications, for example, rocks and debris thrown up by the tires, or over which the vehicle can skid is readily available to tear the relatively soft rubber boot. In off the road heavy machinery applications, not only is there a substantial danger of tearing the boot during a relatively short period of operation, but almost a guarantee of enough debris to cause the joint to fail shortly after it has torn.
For other recreational applications, such as four wheel drive vehicles, all-terrain vehicles, rock climbers, and the like where the universal joints are flexed to their limits because of the uneven nature of the terrain, the constant velocity joints will also be a benefit. But again, there is also the possibility of likelihood of tearing the rubber boot with almost certain failure of the joint to follow, particularly when running through sand, water, and the like.
There are also suggestions in the art to use closer fitting, less flexible covers over constant velocity joints. However, these are typically characterized as complex, often formed of multiple parts, requiring springs or the like to keep them in operation, and having relatively limited flexing capability, well short of the 40 degree flexing capability demanded in some off road applications.